Magnetic core for electrical induction apparatus with reduced magnetic losses



Feb. 9, 1 965 J. M. MQUADE 3,159,235

MAGNETIC SURE FOR ELECTRICAL INDUCTI QN APPARATUS WITH REDUCED NAGNEITIC 3E3 F1186 April I7 19 United States Patent Ofiiice 3,169,236 Patented Feb. 9,1965

MAGNETIC CORE FOR ELECTRICAL INDUC- TION APPARATUS WITH REDUCED MAG- NETIC LOSSES James M. McQuade, Pittsfield, Mass., assignor-to General Electric Company, a corporation of New York Filed Apr. 17, 1961, Ser. No. 103,226

4 Claims. (Cl. 336-,-218) This invention relates to electrical induction apparatus, such as transformers, dynamos and the like, having members made from grain oriented silicon steels, and to a process for reducing magnetic losses in the apparatus by improving the cross-grain magnetic properties of the steels.

Magnetic steels having from about 2 to 6% silicon are widely used as core materials in electrical induction apparatus. Several well known commercial processes may be employed to produce an oriented grain structure in the steels so that the magnetic properties of the steel, such as watts loss and magneto-strictive strain, are rela tively low in the direction of grain orientation. Such commercial processes usually include the following steps: A ferrous alloy containing up to about 6% silicon and relatively minor amounts of impurities is cast intoingots; hot worked, usually by rolling as a continuous strand; and then subjected to varying schedules of usually unidirectional cold rolling. The material is then heat-treated by several combinations of treating cycles which cause primary recrystallization of the grains, decarburization, secondary recrystallization of the grains, and purification by removal of sulfur and, other impurities. orientation of the magnetic steels produced by the above type of process is usually referred to as the (110) [001] type in the standard notation by Millersindices. This designation is intended to indicate that the [0011 direction of the crystals is parallel to the rolling direction and the (110) plane is parallel to the rolling plane. Considering the crystals as cubes, the designation indicates a cube on edge position of the crystal in the plane of the sheet.

Although the above described steels have excellent steel.

rent losses. Also, when a plurality of such laminations are stacked in Contact with each other during high temperature anneal of the steel, it is desirable that they be coated with, a material that prevents them from sticking together. A, widely practiced commercial process for applying an insulating separator coating to silicon steels has been to apply a slurry of magnesium oxide (MgO) t0 the surface of the steel and then dry in air to evaporate water, thus leaving a powdery mixture of magnesium oxide and magnesium hydroxide [Mg(0H)z] on the Such a coating was normally applied before the final heat treating of the steel during which secondary recrystallization and purification .occurecl. When the steel was given its final heat treatment, a chemical reaction took place in which the water of hydration of a the magnesium hydroxide was driven off and all of the magnesium oxide was available to. combine with the siliwatt losses and magnetostriction in the cross-grain direc- The grain magnetic properties in the direction of grain orientation,

For example,

netic flux traveling through the core laminations must change directions and travel across the grain, high watt losses and undesirable magnetostrictive effects occur in the areas where the flux travels across the grain. A particular example of such apparatus is a transformer having a rectangular core in which the flux travels with the grain through the major portion of the core but must travel across the grain at the corners of the core.

Various solutions to the problem of poor cross grain properties have been proposed by the prior art. These solutions include providing inserts with different grain orientations at the corners of the core, or subjecting the oriented steels to special processes whose only function. is to alter the grain structure of the steelfor improving its cross grain magnetic properties. These solutions are unsatisfactory in that they unduly increase the cost of the apparatus.

In electrical induction apparatus having flux carrying members made from stacked laminations of grain oriented steels, it is often necessary that the individual laminations be separated by an insulatingcoatingto reduce eddy curtion of the steel.

v Accordingly,- it is an object of the invention to provide electrical induction apparatus having flux carrying members made from grain oriented silicon steels with improved magnetic properties in selected areas where flux travels across the grain during energization of the apparatus. l -A further object of the invention is to provide a process for improving the cross grain magnetic properties of grain orientedfsilicon steels used in electrical induction apparatusby coating the steels in the areas where flux travels across the grain with a film that also serves as non-sticking electrical insulation.

, 'Afurther object of the invention is to provide a process for controlling'theproperties of silicon steel laminations in electrical induction apparatus at selected areas of the laminations where flux travels across the grain.

Other objects and advantages of the invention will become apparent from the description and claims which follow.

Briefly stated, according to one aspect of the invention, the cross-grain magnetic properties of grain oriented sili-- consteel used in flux carrying members of electrical in- ;duction apparatusmay be improved in. selected areas where the magnetic flux will travela'cross the grain by bondinga coating consisting essentially of beta calcium orthosilicate to the selected areas only. According to another aspect of the invention, a magnetic core for electrical induction apparatus made from a plurality of laminations of grain oriented silicon steels may have its cross grain magnetic properties improved in the areas where flux will travel across the grain by coating the laminations in the selected areas with beta calcium orthosilicate; the surface, area of the laminations where flux travels with the grain should be free from the coating. The coating also serves the function of electrically insulating and separating the laminations. Thus by practicing my invention a. coatingthat serves a triple function is provided for grain oriented steels used in electrical induction apparatus.

The invention will be better understood from the following description taken in conjunction with the accompany drawing, in which: I

FIGURE 1 is an elevational view, partly broken away, of a transformer in which the invention maybe embodied; and FIGURE 2 is'a view of a magnetic core formed of straight laminations to which the invention is applicable.

Referring now to the drawing, and particularly to FIG- coils 4, 4'.

As shown in greater detail in FIGURE 2, core 3 typically comprises legs made up of stacks of straight laminations 5 of silicon steel and yokes made up of stacks of similar straight laminations 6. Conventionally, laminations 5 and 6 are so processed that the direction of their grain orientation is along their lengths. In accordance with the invention, only the portions of the laminations which are at the corners of the core 3 are provided with coatings 7 of calcium hydroxide, as represented by the cross-hatching. The remaining portions of the laminations shown by longitudinal parallel lines may be provided with a coating of other materials, as described hereinafter.

My tests have shown that when semi-processed, grain oriented silicon steel is coated with calcium hydroxide, a high strength insulating film can be formed during the final heating treating cycle, and the cross grain watt losses and magnetostriction of the steel decreased, rather than increased as they were in prior art coating processes. The expression semi-processed is intended to designate steels which have been processed to the extent that the type of grain orientation they will have has been fixed, but that have not been given a final anneal during which secondary recrystallization and purification take place. A water slurry of the calcium hydroxide may be applied to selected areas of the steel where the flux is to pass in the cross grain direction during use of the steel in induction apparatus. This may be accomplished, for example, by masking oft the areas it is not desired to coat and brushing the calcium hydroxide slurry on the selected areas. The calcium hydroxide slurry can also be applied to the selected areas by dipping, or by employing coating rolls with recessed surfaces. Other methods of applying calcium hydroxide are by dusting the dry powder on the selected areas, or by electrolytic coating. The steel coated with the calcium hydroxide slurry may then be dried in air for about one minute at about 200 C. to drive off water, and then subjected to the final heat treatment that causes secondary recrystallization and removal of impurities.

The final heat treating cycles for the semi-processed steels utilized in the prior art may vary from about 200 to 1250 C. for from 1 to 8 hours, depending on the particular sequence and range of heating and cooling steps followed. To obtain satisfactory results from my process, however, the final anneal should be carried out at the upper end of the temperature range between about 900 and 1250 C. for from 2 to 8 hours. During the final heat treatment, the water of hydration of the calcium compounds is driven off and a chemical reaction takes place between the resulting calcium oxide (CaG) and silicon dioxide (SiO on the surface of the steel. Chemical analysis has revealed that the reaction product is beta calcium orthosilicate (BCa SiOQ which is firmly bonded to the surface of the steel. The beta calcium orthosilicate coating on the steel has good electrical insulating properties and also has the eifect of decreasing watt losses and magnetostriction strain in the cross-grain direction of the steel. The coating is refractory in nature and thus serves as an excellent separator of stacked laminations during the high temperature anneals.

In the practice of the invention, calcium hydroxide must not be applied to those areas of grain oriented silicon steel laminations where flux will travel in the with-grain direction because with-grain magnetic properties are harmed to the same extent that cross-grain properties are benefitted by the final beta calcium orthosilicate coating. The areas where flux will travel with the grain should have a ditferent coating applied thereto. For example, magnesium silicate coatings produced by the previously described prior art process may be utilized by applying magnesium hydroxide to only the with-grain portions and annealing in the same manner used to produce the beta calcium orthosilicate coating. Thus the cross grain properties of the steel will be benefited rather than harmed,

4 and an insulating coating obtained without necessitating a special annealing cycle.

For the purpose of illustration, the following specific examples of processes in accord with my teachings are presented below:

EXAMPLE 8 In a laboratory experiment, cross grain Epstein strips were prepared from semi-processed 3 At% silicon steel having a [001] type of grain orientation; the steel was semi-processed in that it had not been given a final grain growth and purification anneal. The strips were coated with a water slurry of calcium hydroxide weighing about .02 ounce per square foot of coated surface and having a thickness of .2 mil per side. The strips were then dried in air to drive off the water. The strips were then annealed for 8 hours in a dry hydrogen atmosphere at 1175 C. The resulting coating of beta calcium orthosilicate had a thickness of about .1 mil. Epstein strips from the same lot of steel were also coated with magnesium hydroxide and processed in the same manner; the resulting coating was magnesium silicate. Uncoated strips from the same lot were processed the same way to provide control samples. The uncoated control samples originally had a thin film of aluminum oxide (A1 0 on the surface thereof to act as a separator coating; the film did not bind to the samples and had no insulation value. This coating fell ofi after the heat treatment leaving the finally treated strips uncoated. All strips were then tested individually for watt losses and magnetostriction, and the results are presented below in Table I. The values given are the average for seven samples.

Table I clearly shows that the cross-grain core losses of the strips coated with beta calcium orthosilicate decreased about 10% when compared with the uncoated samples, while the magnetostriction strain decreased over 50%. The strips coated with magnesium silicate showed an average increase in core losses across the grain of over 10% when compared with the control samples, while the magnetostriction strain increased about 20%.

EXAMPLE II A dilferent lot of semi-processed 3 /4% silicon steel having the same orientation as in Example I was cut into cross-grain Epstein samples which were coated with calcium hydroxide, and magnesium hydroxide and then processed in the manner described with reference to Example 1, except that the samples from this lot were annealed at about 980 C. for eight hours. The beta calcium orthosilicate coatings were about .1 mil thick. Uncoated control samples were also prepared. All samples were then tested for core loss in packs of 20, rather than being tested individually as in Example I. No tests were made for magnetostriction strain. The results of the tests are given below in Table II.

Table II shows that the samples coated with beta calcium orthosilicate improved in cross-grain losses, but that the improvement was not as great as at the higher temperature annealof Example I. The samples-coated with magnesium silicate showed a slight increase in crossgrain core loss.

To obtain maximum benefit from the beta calcium orthosilicate coatings, the final grain growth and purification anneal should be carried out atthe higher end of the temperature range used for such anneals. The preferred temperature range is from 1100 to 1250 C.,'and' satisfactory results have been obtained in as little as two hours in that range. Cross-grain magnetic properties can be improved at temperatures down to about 900 C. in longer time cycles.

The beta calcium orthosilicate coatings produced from calcium hydroxide in accord with my invention satisfy the normal requirements for electrical insulation in that the coatings had Franklin values in the range of from .2 to .4 ampere, as determined by the standard Franklin test for determining the insulation valve of this type of coating; in this test a reading of l ampere represents no surface insulation and reading of amperes represents perfect insulation. Coatings with Franklin insulation values of .4 ampere and below are generally considered satisfactory 'for silicon steels used as flux'carrying members in electrical induction apparatus. The beta calcium orthosilicate films also have good adherence to the silicon steels in that the samples could be flexed without the coating becoming loosened and falling off.

It has thus been shown that by the practice of my invention, magnetic losses in electrical induction apparatus can be reduced by controlling the properties of grain; oriented silicon steels used to make flux carrying me'm bers in selected areas where the flux travels across the grain during energization to the apparatus. I have shown that this can be accomplished by bonding a coating of beta calcium orthosilicate to the selective areas where the flux will travel in the cross-grain direction. This coating also has good insulating and separating properties and thus serves the triple function of insulating the material, preventing sticking at high temperatures, while also enabling control of magnetic properties in desired areas. Consequently, the necessity for a separate treating cycle having the sole function of improving the cross-grain properties of the steel is not required because the step of coating with an insulating material before the final heating treating anneal is normally practiced in commerical production of these steels.

It will be understood, or course, that while the forms of the invention herein described constitute preferred embodiments of the invention, it is not intended herein to describe all the equivalent forms or ramifications thereof. It will also be understood that words used are words of description rather than of limitation, and that various changes may be made Without departing from Table ll CROSS-GRAIN MAGNETIC PROPERTIES 7 Core Loss at Coating 15.000 gauss (Watts/lb.)

Uncoated .Q 1.01 Beta calcium orthosilicate" Magnesium silicate 1. 98

the scope or spirit of the invention herein disclosed, and it is aimed in the appended claims to cover all such changes as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In an electrical transformer, a substantially rectangular magnetic core comprisinga plurality of stacked laminations of grain-oriented silicon steel, the direction of orientation of the grains being substantially parallel to the long sides of said laminations, there being magnetic flux passing through said core'when the transformer is in operation, said flux traveling in the direction of grain "orientation'through the major portion of saidcore, and

said flux traveling in the cross-grain direction at the corners of said core, an electrical insulating coating consisting essentially of betacalcium-orthosilicate bonded tothe surface of said laminations only at'the corners where flux travels in the cross grain direction, whereby core losses and magneto-strictive strains caused by fiux traveling across the grain when said apparatus is energized are reduced.

2. In an electrical transformer, a substantially rectangular magnetic core comprising a plurality of stacked laminations of grain-oriented silicon steel, there being magnetic flux passing through saidcore when said apparatus is in operation, said flux traveling in the direction of grain orientation through the major portion of 7 9 and a different coating bonded to the surface of said laminations where flux travels with the grain, whereby core losses and magnetostrictive strains caused by flux traveling across the grain when said apparatus is energized are reduced. r

3. Electrical induction apparatus having a member comprising a sheet of annealed grain-oriented silicon steel containing up to about 6% silicon, there being magnetic flux passing through said sheet when said apparatus is in operation, said'flux traveling in the direction of grain orientation in one portion of said sheet, and said fiux traveling in the cross grain direction in another portion of said sheet, an electrical insulating coating consisting essentially of beta calcium orthosilicate bonded only to the surface of the portion of said sheet where flux travels in the cross grain direction, whereby core losses and 'magnetostrictive strains caused by flux traveling across the grain when said apparatus is energized are reduced.

4. Electrical induction apparatus having a magntic core comprising'a plurality of laminations of annealed grain-oriented silicon steel containing up to about 6% silicon, there being magnetic flux passing through said core when saidapparatus is in operation, said flux traveling in the direction of grain orientation in one portion of said core, and said flux traveling in the cross-grain direction in another portion of said core, an electrical insulating coating consisting essentially of beta calcium orthosilicate bonded only to the surface of the portion of said laminations where flux travels in the cross-grain direction and a different coating bonded to the surface of said portion where flux travels with the grain, whereby core losses and magnetostrictive strains caused by flux traveling across the grain when said apparatus is energized are reduced. 7

References Cited by the Examiner UNITED STATES PATENTS 2,904,875 9/59 McBride et al 291'55.61 2,909,741 10/59 Arntzen et a1 336219 2,966,725 1/61 Parker et al. 29-155.61 3,029,403 4/62 Krueger 336-219 JOHNF. BURNS, Primary Examiner. ORIS L. RADER, Examiner.- 

1. IN AN ELECTRICAL TRANSFORMER, A SUBSTANTIALLY RECTANGULAR MAGNETIC CORE COMPRISING A PLURALITY OF STACKED LAMINATIONS OF GRAIN-ORIENTED SILICON STEEL, THE DIRECTION OF ORIENTATION OF THE GRAINS BEING SUBSTANTIALLY PARALLEL TO THE LONG SIDES OF SAID LAMINATION, THERE BEING MAGNETIC FLUX PASSING THROUGH SAID CORE WHEN THE TRANSFORMER IS IN OPERATION, SAID FLUX TRAVELING IN THE DIRECTION OF GRAIN OREINTATION THROUGH THE MAJOR PORTION OF SAID CORE, AND SAID FLUX TRAVELING THE CROSS-GRAIN DIRECTION AT TEH CORNERS OF SAID CORE, AN ELECTRICAL INSULATING COATING CONSISTING ESSENTIALLY OF BETA CALCIUM ORTHOSILICATED BONDED TO THE SURFACE OF SAID LAMINATION ONLY AT THE CORNERS WHERE FLUX TRAVELS IN THE CROSS GRAIN DIRECTION, WHEREBY CORE LOSSES AND MAGNETO-STRICTIVE STRAINS CAUSED BY FLUX TRAVELING ACROSS THE GRAIN WHEN SAID APPARATUS IS ENERGIZED ARE REDUCED. 